Well, we know from even within the Star Trek universe that it's entirely possible to stall on a plateau for thousands or even millions of years and make very slow or even no progress. Take the Voth for example. They seem 'only' about a thousand years more advanced than Voyager, but they're 65 million years old. You would expect them to have reached Q level powers by now - even if not by evolution, by technology. In the case of the Voth, it was clearly also their own doing (Doctrine), but it could for example also be a critical breakthrough technology that keeps eluding the scientists. Something like achieving transwarp, which the Federation has tried unsuccesfully for at least a hundred years by 2370, but then a much harder technology with a much higher threshold still. I see no reason why such a stall couldn't happen to the Federation or their remains.
The comparison doesn't make much sense to me because the Voth (for example) never really 'tried' to develop TW or other technologies after settling in the DQ and their doctrine was one of the primary factors to blame for the stagnation in question. Their development of TW seemed to be mostly 'accidental' and took a large amount of time to incorporate exactly because of their Doctrine (which had to be adjusted in the face of this new technology).
We also don't know how long early Voth spent in space reaching the Delta Quadrant from Earth. Were they in stasis for most of the time? Did they use Warp technology like early SF, or have they used sublight propulsion coupled with relativitistic effects? For all we know, the Voth may have spent millions of years travelling to the DQ (making only small updates to their technology in the process - especially if they were in stasis) and only recently (n the last several thousand years - possibly even 10 000 years) settled and developed there.
That could explain why we hadn't seen millions (or hundreds of thousands... or heck even thousands) of years worth from progress from the Voth during their voyage to the DQ.
Their Doctrine also came into existence and ended up slowing things down further.
The UFP doesn't have nearly as many (or practically any) hangups in terms of developing technology and emphasizes exploration (which would have pushed it to advance further and faster... beyond the Milky Way even).
Sure, UFP banned certain technologies due to ethical concerns or potential for environmental damage, but nothing on the scale that would prevent them from finding a way to make TW (or other faster than Warp technology) work for their ships (like it was demonstrated that they do but required more R&D to get rid of some initial bugs) or pursuing alternative energy sources which didn't rely on dilithium (which in turn needed heavy mining and recrystalization)... if anything, the UFP would have developed these technologies much faster than the Voth because they would have kept pursuing and actively developing them for the most part (but the way some live actions portrays UFP is that it ABANDONS certain tech after a failed first attempt, which of course is utterly nonsensical - science relies on many previous failures so you can learn from them and then devise safer meansmoving forward - heck, if a VOY crew was able to develop V2 of QS technology in only 4 - 6 months which was thousands of times faster than V1 and no longer had issues with quantum stresses, then this gives you an indication the UFP with all its resources would be able to do FAR more with many other technologies in a similar amount of time or a year or two after at most).
In any event, the amount of effort UFP puts into technological development vs the Voth is like night and day.
And even if a tech wasn't immediately viable... it probably would have become in the next decade or few.
The UFP frequently encountered more powerful species and plethora of anomalies or natural phenomena that endangered their ships and crews... they would have had the motive and dedication (and resources) to make far more breakthroughs and in turn make their ships far more resilient and safer (in the short and long term).
Also, the Voth are isolationists... the UFP readily accepts/absorbs different species, perspectives and their science and technology and incorporates/evolves it from there (because its based on free pursuit and sharing of knowledge, ideas, resources, etc.).
Plateus would only happen if you have social hangups and 0 desire to advance technology... UFP may have banned or restricted some technology, but nothing of the kind was mentioned to have been the cause for not developing much faster propulsion, superior shielding, weapons or energy sources (which would have been necessary to continue exploration of the galaxy and the universe at large - especially because SF officers always loved the prospect at being able to explore beyond the Milky Way), and they still lived in a fairly dangerous galaxy and universe and faced many unknowns on a continuous basis (that alone would have been sufficient to prevent an organisation like UFP from stagnating).
If anythign, a measily bad explanation was given that some technologies proved 'unreliable'. I'm sorry... they were more than reliable for alien species who developed them and used them on a regular basis without problems... and even for SF ships they worked 'temporarily' upon first introduction (and its expected you'll have some 'bugs' to work out initially)... but to think that in 800 freaking years this hadn't changed whatsoever is ludicrous.
Any of the 'kinks' would have realistically been worked out within 10 to 30 (50 at most) years after initial introduction of a technology. SF would also adjust their designs to suit the new tech if the existing designs demonstrate to be less ideal - but hey, even modifying existing ships with new tech that allows 'hops' in crossing large interstellar distances would have changed response times for SF ships for the better.
Say that QS v1 continued to have issues in terms of quantum stresses impacting the hull... fine, just limit it to using it for say 30 or 45 mins at a time and adjust existing ships self repair tech to accommodate as much as it is possible to mitigate those quantum stresses (this can lead to other breakthroughs in the future) - you get faster than Warp tech which you can use for just under an hour at a time with some downtime without having to use or synthesize Benamite crystals in the process (whereas those can be reserved for QS v2 and larger jumps once the phase variance issue was resolved - and that can be done by developing better computer systems - VOY's computer was at the time not fast enough to calculate and adapt to changes in the Slipstream threshold - and benamite crystals would also eventually be able to be synthesized far faster and probably even recrystalized - SF managed to find other solutions to existing technology to augment it... so this wouldn't be that different).
